The present invention relates to the use of heat from geothermal fluids, including steam and brines, and is particularly concerned with steam entering into an underground reservoir into which oil shale is placed and retorted. The geothermal source can be natural or man made (as by atomic explosion underground and/or by injection of water or other heat exchange fluid into an underground reservoir or a deep well and/or by injection of a heat exchange fluid, e.g., water into a spent shale in situ retort).
Various methods are known for utilizing the geothermal steam or brine for electrical power generation. Thus, according to one procedure, the hot geothermal brine is directly flashed and the resulting flashed steam is then expanded through a turbine for electrical generation. According to another mode of procedure, a tube and shell heat exchange apparatus is employed for indirect heat exchange contact between the hot brine on one side and water or a working fluid on the other side, and the heated steam or working fluid is then passed to the turbine for generating power. Such methods are described for example, in Geothermal Energy Utilization by Edward F. Wahl, John Wiley & Sons, New York (1977).
According to a third method, direct contact heat exchange is provided between the geothermal brine and an immiscible (e.g., isobutane) working fluid, and the working fluid is expended through a turbine to produce electrical energy. Illustrations of the latter system are those disclosed in U.S. Pat. No. 3,988,895 to Sheinbaum, application Ser. No. 589,068, filed June 23, 1975, now abandoned by S. F. Woinsky, application Ser. No. 873,264 of E. F. Wahl et al, filed Jan. 30, 1978, now U.S. Pat. No. 4,167,099 and application Ser. No. 50,868 of P. Sadkukhan, filed June 21, 1979, now U.S. Pat. No. 4,272,961 which applications are incorporated herein by reference.
Others have suggested various procedures for replacing spent oil shale materials in the voids formed within the subterranean oil shale deposit. For example, U.S. Pat. No. 3,340,693 suggests mining oil shale and backfilling at least some of the mined out spaces with porous masses of particles through which a suitable reagent is flowed to consolidate the masses. U.S. Pat. No. 3,459,003 suggests mining and retorting oil shale, pumping a slurry containing some of the spent shale into the mine to form a porous mass, thermally converting the remainder of the spent shale to a cement and pumping a slurry of the cement into the mine to fill the pores of the porous mass of spent shale. U.S. Pat. No. 3,588,175 suggests dividing a zone being mined into a number of production levels, mining concurrently on several levels while leaving relatively weak pillars between adjacent stopes, then promptly providing bulkheads and pumping slurries of spent shale into the emptied stopes while removing water to allow the spent shale masses to compact under their own weight.
U.S. Pat. No. 4,081,968 teaches mechanically compacting spent shale to a volume at least substantially as small as that of the in situ volume of the portion of oil shale from which the spent oil shale was obtained. This compacted spent shale is disposed within a mined out portion of the subterranean oil shale.
Still others, due to the problems normally encountered in surface retorting (e.g., cooling and disposal of spent shale), have used in situ retorting of oil shale as a possible means to recover hydrocarbon products from oil shale. Some in situ operations have a retorting zone or gallery formed within the oil shale deposit by first mining out a portion of the shale to create a cavity and then rubblizing the surrounding shale into the cavity by means of explosives or the like. The necessary heat for retorting is then applied to the rubblized shale either by in situ combustion or by circulating externally heated gas therethrough. One such method is taught in U.S. Pat. No. 4,018,280.